Effect of H2O and O2 on NOx adsorption and stability over Pd/SSZ-13 low-T adsorbers: mechanistic aspects investigated by operando FT-IR spectroscopy

In this study, by combining microreactor experiments and operando FT-IR spectroscopy, the effect of H2O and O2 on the NOx storage and release characteristics over a Pd-doped SSZ-13 zeolite PNA catalyst is investigated. The concentration of the surface species could also be determined. It is found that in the presence of water and O2, NO is stored on Pd sites in the form of nitrosyls on hydrated and anhydrous Pd+ and Pd2+ centres through a complex redox pathway involving the reduction of Pd2+ to Pd+ and the evolution of NO2 according to the following stoichiometry: 2PdOH++3NO→2Pd+NO+NO2+H2O Nitrosyls stored over Pd+ species show higher thermal stability with respect to those formed over Pd2+. More than 70 % of the Pd sites are involved in the storage and their amounts are not significantly affected by temperature in the range 80–150 °C. Oxygen and mostly water affect the occurrence of the NO storage/decomposition pathways. O2 does not affect the occurrence of the storage redox pathway but impacts the stability of adsorbed nitrosyl species upon heating, through the reoxidation of Pd+ to Pdⁿ+ at high temperatures thereby decreasing the stability of the adsorbed nitrosyl species. At variance, water has a profound impact on the storage pathways in that its presence greatly facilitates the reduction of Pd2+ to Pd+ and hence formation of NO₂ according to the previous redox reaction. Besides, water also inhibits the direct NO oxidation to NO₂ over Pd sites. This leads to completely different NO2 evolution during the NO storage in the presence/absence of water. Moreover, water also blocks the BAS (Brønsted acid sites) of the zeolite thus preventing NO storage as NO+, that represents a significant NO storage under anhydrous conditions. The NO+ species formed in the absence of water tend to desorb already at low temperature; at variance water presence enhances the stability of the nitrosyls by favoring the formation of more stable Pd+ nitrosyls. Accordingly, the NO release during the catalyst heating is observed at much higher temperatures in the presence of water.